Phase I - Topic 307 There is a critical need to predict the efficacy and potency of adoptive cellular immunotherapies (ACIT) as these therapies gain traction in treatment of cancer with over 90 clinical trials planned this year. A prominent approach in ACIT involves introducing activated T cells targeted towards specific cancer cells into patients to either directly kill the cancer cells or secrete factors to activate other immune cells to kill the cancer cells. Unfortunately, there are currently no FDA approved products in the market for non-invasively imaging activated T cells. We are proposing to develop [18F]F-AraG, a PET imaging tracer, to visualize and localize activated T cells in animal models or patients receiving ACIT for cancer. Initial studies are very promising so in this proposal we intend to 1) Analyze relative specificity of the tracer 2) Determine pharmacokinetics of the tracer in mice models 3) Perform tracer sensitivity studies in preparation for a phase 2 grant which will include clinical studies. This project's success will not only lead to use of this probe in development of ACIT for cancer, but will also be extremely valuable as a clinical management tool to allow physicians to determine patients autoimmune response and aid in treatment decisions.